Sharpening Carving Tools

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Woodcarving
By
NUMBERS
Pt.2 — Sharpening
by Mike Davies
S
harp tools are essential for effective
woodcarving. Even though many
manufacturers of woodcarving tools
supply their products ready sharpened, I
would recommend that you spend time,
right from the very start, learning how to
maintain the cutting edge to get the best
results from your tools.
If your tools are not razor sharp, it will
make learning to carve difficult and frustrating, so I can’t stress how important it is
for you to master this.
To begin, let us consider how carving
tools are manufactured. The steel blades
of carving tools undergo a heat treating
process called tempering. This process
makes them very hard and durable.
However, if the tools are heated excessively during the sharpening process, they
may lose their tempered quality and
become brittle.
Many woodworkers have bench
grinders in their workshops which are
very effective at removing metal quickly.
The one drawback, however, is that if
Photo.2: A blade after being ground at
90° to the centre line of tool
Photo.3: Presenting the blade to the
grinder wheel
used incorrectly, they can also burn your
tools very quickly.
Re-grinding your tools should be a last
resort, as the majority of maintenance on
well-kept tools can be done by honing the
cutting edge with bench stones and
strops.
There are many different sharpening
systems available on the market and
many will produce razor sharp results.
However, regardless of which system you
choose, you will need an understanding
of the basic sharpening principles.
Sharpening Angles
There are two main angles to consider
when sharpening your carving tools. The
first is the blade angle, which is used to
describe the angle of the cutting edge in
relation to the centre line of the tool
(Photo.1).
For the majority of carving tools, the
blade angle is kept at 90° to the centre
line of the tool. The cutting edge needs to
be completely flat and the outer tips of the
cutting edge should be intact, not rounded over.
The second angle to consider is the
cutting edge angle or ground edge angle
(Photo.1a). The cutting edge angle of your
carving tools will vary anywhere between
20° to 30°.
This angle should vary in relation to
the density of the timber that you are carving. A 20° angle will give you a far keener
edge but it will be more susceptible to
damage. For smaller finishing tools, or if
you carve mostly softwoods, a 20° angle
would be ideal.
If you were to use a mallet to remove
large quantities of hardwood, such as
Jarrah, then a greater angle closer to 30°
would be required to maintain a functional cutting edge. As a general rule, the
Photo.4: Multiple flats caused by rotating
the gouge unevenly while grinding
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Australian Woodworker
September/October ’13
Photos.1 & 1a: Blade angle (90°) left and
cutting edge angle (20-30°) right
harder the timber, the greater the angle
should be. For a multipurpose tool aim for
25°.
Incorrect sharpening techniques can
lead to defects such as chips along the
cutting edge, rounded tips to the cutting
edge and the blade angle is no longer at
90° to the centre line of the tool. Where
the cutting edge has become rounded
rather than a clean straight angle, this is
caused by poor honing techniques.
Traditional Sharpening Techniques
With a badly damaged tool, the only
way forward is to regrind the blade angle
and then the cutting edge angle.
When using a dry wheeled grinder, it
is essential to keep the steel cool at all
times. This can be done by applying minimal pressure with the blade onto the
grind stone. It is also important to dip the
tool in water after each pass on the
Photo.5: A correctly ground cutting edge
angle on a gouge
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Photo.6: The author prefers to make the
honed angle as close as possible to the
ground angle
machine. When using a grindstone,
always wear eye protection, keep your
fingers away from moving components
and make sure all loose fitting clothing is
tucked away from the machine.
Good lighting is also important so that
you can see your progress as you shape
the cutting edge.
Ensure the grinding wheels on your
machine are clean, flat across the face
and running true. With the grinder
switched off and unplugged, adjust the
toolrest so that it is close to the wheel and
rotate the wheel by hand to make sure it
runs at a consistent distance from the tool
rest. If not, the wheel will need dressing.
The first task is to grind the blade
angle. The goal is to obtain a perfectly
straight line at 90° to the centre line of
tool. Note in Photo.2 how all the chips in
the cutting edge have been ground away
and the tips of the tool are clean and
intact. You will also see how a flat has
been created along the cutting edge. This
is often referred to as a flat-line or line-oflight.
The next task is to create the cutting
edge angle. Some carvers make a series of
templates to help grind to a precise angle,
but the simplest approach is to draw several angles on a piece of paper and use
them as a guide.
Use your forefinger and thumb to
pinch the blade of the carving tool, so that
your forefinger forms a stop against the
toolrest of the grinder (Photo.3). The trick
here is to maintain the same grip on the
tool from the start of the grind to the finish. In this manner your finger will help to
achieve a consistent angle.
Remember not to change your grip on
the tool when dipping the blade in water.
This technique applies to both flat cutting edge chisels and curved shaped
gouges. If you are grinding a curved
gouge, then rotate the tool as you pass the
blade across the grind stone. Try to avoid
grinding numerous flat spots around the
edge (Photo.4). You need to create a consistent ground edge angle around the circumference of the gouge (Photo.5).
Continue grinding a consistent angle
until you reach the stage where the flat
line is almost completely removed. The
goal is to have a consistent flat line that is
less than 0.1mm in thickness along the
cutting edge. Avoid grinding through the
flat line as this will create a tool with a
wavy cutting edge.
Honing
Having set the cutting edge angle, the
next step is to hone the blade.
There are numerous sharpening stones
available — silicone carbide, natural
Arkansas stones and diamond sharpening
systems to name but a few. Whichever
one you choose, the same techniques will
be required.
I recommend having two stones for
honing. The first can be used for curved
gouges and the second for flat cutting
edge chisels.
There are two schools of thought for
the honing angle. Many like to slightly
increase the honed angle from the ground
edge angle. The benefit is that you can be
confident about putting the cutting edge
in contact with the stone, as opposed to
the back edge of the ground angle (also
called the heel). The downside is that you
can quickly create a convex shape to the
ground bevel and to remove this will
require a fresh re-grind.
I generally prefer to make the honed
angle as close to the ground angle as possible (Photo.6).
Honing Flat Chisels
Position the sharpening stone on your
workbench with its length running away
Photo.7: Honing a flat chisel
Photo.9
Photos.8-10: Left to right — the sliding and rotating movement
required to provide an even hone along the circumference of
the cutting edge of a gouge
Photo.10
Australian Woodworker
September/October ’13
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Photo.11: To avoid an inside bevel, hold the shank flush against
the length of the slip stone when removing the burr
Photo.12: Refining the honed edge with
a slip stone
Photo.13: Running the outside and inside
faces of the cutting edge down a leather
strop
from you. Add a little oil or water to the
surface according to the manufacturer’s
instructions.
Place the chisel onto the stone’s surface at the required angle and slide it back
and forth in a continuous motion
(Photo.7). It is essential that the same
angle is maintained throughout this
process. Continue until you are able to
feel a small ridge of steel that will form on
the opposite side of the cutting edge. This
is called a burr and needs to be removed
using a slip stone.
Honing Curved Cutting Edges
When honing a gouge, fluter or veiner,
place the stone horizontally in front of
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Australian Woodworker
Photo.14: Two test cuts — the one on the left is a perfect polished cut while the one on the right has fine drag marks caused
by the sharpened edge not cutting cleanly
you and add oil or water as appropriate.
Place the gouge at the required angle
on the stones surface and slide it from side
to side along the stone’s length. As you
slide, rotate the handle of the gouge so
that the curvature of the blade receives an
even grind (Photos.8-10). Be careful not
to hone away the corners or tips of the
cutting edge, which need to be kept at a
sharp 90° angle to the shank of the tool.
The shank refers to the entire length of the
steel blade between the handle and the
cutting edge.
If the corners become rounded, then it
is probably due to the gouge being rotated too far. It may be helpful to practise the
sliding motion initially on a flat piece of
softwood.
When using the stone, continue the
process until you can feel the burr on the
inside of the gouge’s curve, opposite the
ground edge angle. This should then be
removed with a slip stone.
Using a Slip Stone
Slipstones are used to remove burrs
and provide a finer cutting edge. The slipstones should be a finer grade than the
bench stone and when working with
curved cutting edges, they must fit inside
the tool profiles.
Hold the slip stone between your fingertips and thumb and rub the length of
the slipstone on the inside contour of the
carving tool. The objective is to remove
the burr.
Some carvers like to form an inside
bevel, though I prefer to avoid this.
Therefore I keep the entire flat face of the
slipstone rubbing against the length of the
September/October ’13
Photo.15: With a wet stone system,
use an angle setting gauge for precise
results
shank (Photo.11).
After removing the burr, you can then
use the slip stone to create a finer cutting
edge. To do this, simply use the flat side of
the slipstone to rub ‘up and down’ the
cutting edge (Photo.12). Be sure to maintain a consistent angle, ie. the honed edge
angle. You do not want to round over the
sharp cutting edge.
Stropping the Blade
The final process involves a leather
strop, similar to one that a barber would
use to refine the edge of a razor.
The leather should be impregnated
with a fine abrasive compound, such as
jewellers rouge. Run the outside and
inside faces of the cutting edge down the
leather strop to produce a polished cutting edge which should produce razor
sharp results (Photo.13).
Testing the Sharpness of Your Tools
Test the sharpness of your tools by
using them on a piece of scrap carving
wood. Try sliding the blade through the
timber, both with the grain and across it. If
you have a razor sharp cutting edge it
should glide effortlessly through the timber, like a hot knife through butter.
Take a close look at the finished cut. If
you have tiny ‘white’ drag marks in the
timber, there is a defect in the blade,
which needs to be removed by further
work with slipstones or even honing.
The goal is a polished cut straight from
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Photo.16: Sharpening with a gouge jig
the blade. In Photo.14 there are two cuts.
The one on the left is a polished cut and
the one on the right has fine drag marks
caused by imperfections in the cutting
edge. The tool that created the drag marks
will require further work until the cutting
edge can produce a clean and polished
surface.
A video presentation on the traditional
method of sharpening carving tools can
be viewed on my Woodcarving
Foundation Skills DVD, available through
The Australian Woodworker Mail Order
Bookshop (details at the end of the article).
Wet Stone Sharpening Systems
A wet stone sharpening system is an
ideal method for grinding and honing
tools to a razor finish. The system carries
water to the blade as it is being ground,
making it impossible to overheat your
tools.
You can produce accurate angles
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Australian Woodworker
Photo.17: Sharpening with a straight edge jig
effortlessly, regardless of the shape of
tool, using an angle setting gauge
(Photo.15).
Most wet stone grinders are available
with a host of jigs (Photos.16 & 17), but it
is important to consider the whole system
when making your purchase. The grade
and quality of the stone and jigs, and the
availability of accessories will affect the
performance and versatility of the sharpening system.
Most wet stone sharpening machines
are fitted with a honing wheel, allowing
you to use the same jigs when switching
from grinding to honing. This allows you
to ensure the honed edge angle is identical to the ground edge angle.
A good quality wet stone grinding system should enable the novice to produce
a mirror finish and a razor sharp cutting
edge, quickly and consistently.
Further Information
September/October ’13
You can view sharpening tutorial
videos for flat chisels, gouges and V-tools,
using a wet stone sharpening system, at
www.recordpower.com.au.
Go
to
Woodworking Machines, Sharpening &
Grinding, Sharpening Systems and then
click on the video’s tab of the WG250.
This series of Woodcarving by
Numbers articles is based around the
tools found in the the Record Power 12
Piece Carving Tool Set. This is available
with the bonus Woodcarving Foundation
Skills DVD and booklet for RRP $132.00
from The Australian Woodworker Mail
Order Bookshop, pp.82-90, Ph: 02 4759
2844 or from Record Power stockists.
Next Issue:
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